Great progress has been made in the field of implantable devices for medical or biological purposes. By far the most common types of implantable devices are the heart pacemakers, but other types of implantable devices have been developed or proposed also, including sensing devices for measuing various medical conditions within the body, metered drug administering devices, muscle stimulators and nerve stimulators. With the great strides that have been made in miniaturization of electronic circuit components, it is possible to provide increasingly sophisticated and varied control and monitoring functions within an implantable device. These advances have made it important to provide an efficient means for transmitting data from within the implanted device to be received and sensed outside the body. The data to be thus transmitted can represent diagnostic data concerning sensed medical conditions within the body, or it can represent data indicative of the operating condition of the circuits within the implanted device itself. One example of this latter category of data can be the important function of transmitting signals indicative of the amount of life remaining in the battery or other power source which powers the implanted device.
It has been proposed previously to use sound or ultrasound waves for transmitting signals from a device implanted within the body. One prior art example is proposed in U.S. Pat. No. 3,672,352 to Summers, which discloses in general form an implantable device having a sensor and a signalling device. A number of different methods for signalling are suggested, including an audible signal. Another system is proposed in U.S. Pat. No. 4,041,954 to O'Hara, which also discloses sensors and various types of energy signal transmitting means, including sound or ultrasonic waves. It is recognized in the prior art that energy from the battery or other energy source for the implanted device must be conserved for long life, and that energy usage by the transmitting means must be kept to a minimum. Accordingly, both of the abovementioned prior art references suggest the use of circuits requiring external microphones for picking up the weak sound signals generated. To further conserve energy they suggest means for supplying energy externally of the body to within the implanted device, or means for inhibiting the implanted signal transmission means until activated externally by a magnet, pressure switch, etc. The weak acoustic signals thus provided by the prior art patents mentioned above, together with the microphones and possible external energization or activation means limit their applicability to testing under controlled conditions, as in a laboratory, hospital, or a doctor's office.
In contrast, the present invention provides audible signalling means for an implanted device which, while operating with very low drain on the energy source of the implanted device, provides an audible signal which may be heard externally of the body with the unaided ear. This feature makes it very useful for classes of devices intended for implantation in human patients, and where a warning must be given to the patient in case of problems associated with a medical process or condition being monitored or treated. For example, the alert signal can be used with a device for measuring blood pressure, in which case the alert signal would prompt the user to take corrective action, such as taking medication or calling his physician. The invention can similarly be used with any physiologically measurable parameters of interest.
As mentioned above, the invention is especially well adapted to providing a warning to the patient when the battery or other energy source in an implantable pacemaker approaches the end of its life. Because of the obvious undesirability and risk to the patient's health should failure of the energy source for a pacemaker occur, a great deal of effort has been expended in prolonging the life of pacemaker batteries, and also in solving the problem of deciding when to replace them. Since replacement involves a surgical procedure, even though not a major one, it is obviously desirable to avoid unnecesary replacement. However, since the useful life of the battery depends upon many variables such as the duty cycle of the device, it is difficult or impossible to predict. Thus, the solution of periodic replacement of the battery with an interval being determined to avoid a predetermined risk of battery failure results in unnecessarily frequent replacement in the vast majority of cases. A method for indicating approaching battery depletion is described in U.S. Pat. No. 3,842,844, wherein circuitry is provided for increasing the pulse width of the output signals, and decreasing the pulse repetition rate, as the battery becomes depleted. Observation of the pulse width and repetition rate, as compared with the known voltage properties of an aging battery, can serve as an indication of the time to replace a pacemaker. However, it cannot be assumed that the patient will necessarily become aware of these indications, and practical reliability may therefore depend upon measurement of these parameters in the context of the doctor's office, a variable beyond the control of the device manufacturer.
Other methods, as disclosed in U.S. Pat. No. 3,618,615 and U.S. Pat. No. 3,713,449 have been proposed which allow an attending physician to observe the response to certain pulse parameters to determine battery life. In general, these methods have the disadvantages of requiring interrogation of the unit by a magnetically operated switch and instruments to read operating parameters. These systems are therefore dependent upon a physician to make the determination, and they will not pick up premature battery failures under normal sampling plans.
Other systems proposed in U.S. Pat. No. 3,783,877 use an auxiliary battery in the implanted device and a switching network that brings it into use when the voltage in the primary battery drops to a predetermined level. Either a variation in the pulse rate or the provision of an additional set of electrodes remote from the heart are provided to indicate switchover to the secondary battery. This system has the disadvantage of requiring two separate batteries where space is at a premium within the pacemaker. Further, the provision of a second set of electrodes provides another complexity that is not justified in all cases.